Improvement in the Weldability and Mechanical Properties of CuZn40 and AA1100-O Dissimilar Joints by Underwater Friction Stir Welding

Friction stir welding (FSW) of brass (CuZn40) and aluminum alloy (AA1100-O) carried out in the open air (AFSW) is affected by several weld flaws, poor heat affected zone (HAZ), and inter-metallic compounds (IMCs), which deteriorate the weld joint quality. This research work investigates the viability of aluminum and brass joints prepared in underwater friction stir welding (UFSW) at different values of plunge depth (PD) and rotational speed (N). In this study, an attempt has been made to characterize the microstructural changes and several types of weld flaws found in FSW of dissimilar joints in the air (AFSW) and water mediums (UFSW). The microstructural properties, strength and micro-hardness of the weld joint attained in AFSW and UFSW at different values of PD and N are calculated and a comparative study is presented here. The present investigation revealed that the presence of crack, tunnel, and void defects of AFSW is significantly minimized in UFSW. The parametric combination, PD and N of 0.2 mm and 1400 rpm produce defect-less joints at both the welding mediums. Scanning Electron Microscopy (SEM) images revealed the formation of IMCs at the joint interface. Energy Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD) results confirmed the presence of Al4Cu9, Al2Cu, AlCu3, and AlCu4 with some amounts of Cu5Zn8, CuZn5, and CuZn. The lower peak welding temperature in UFSW reduces the weld flaws and formation of thick IMCs. Additionally, finer grains were obtained in the weld zone due to faster cooling rates of UFSW. As compared to AFSW, improved hardness is found in the weld cross-section of UFSW. Fractography of the broken samples of weld joint revealed ductile mode of failure at both the welding mediums, with fine and ultrafine dimples in UFSW. The UFSW samples revealed a reduction in the weld flaws and the thickness of IMCs and improved the hardness of the weld joint, which ultimately enhances the overall quality of the weld joint.